A conventional wiper system disposed on a vehicle has a selection of wiping speeds for wiping operation such as an intermittent mode, a low speed mode, a high speed mode, a stop mode (i.e., an off mode) and the like. A driver of the vehicle chooses an appropriate operation mode according to an amount of rain falling on a windshield of the vehicle (refer to a Japanese Patent Document JP-A-2002-283968).
The conventional wiper system includes a rain detection system to automatically control the wiping operation. The rain detection system picks up the amount of rain in a detection area on the windshield. The detection system uses a light source (e.g., a light emitting diode (LED)) coupled with a sensor (e.g., a photo diode (PD)) to intermittently pick up a light reflected from a raindrop. The amount of rain on the windshield is determined by measuring a reflection rate of the light against the emitted amount of the light from the light source.
The reflection rate is determined, for example, by comparing a measurement of an output voltage from the PD in a rain against a measurement of an output voltage from the PD in a fine weather.
The rain detection system may include a prism for redirecting a light from the light source and to the sensor, and the prism may have a heater for preventing condensation of water in an atmosphere from forming thereon.
The conventional wiper system automatically increases a wiping speed when the amount of rain on the windshield increases. That is, the operation mode is changed from an intermittent operation mode (INT mode) or a low speed mode (LO mode) to a high speed mode (HI mode). However, the conventional wiper system mistakenly determines condensation of water on the windshield as an increased amount of rain when the vehicle enters a space such as a tunnel or the like. In this case, the conventional wiper system needlessly changes the operation mode of the wiper system to the HI mode for the sake of coping with the mistakenly detected increased amount of rain.
Further, the conventional wiper system causes discomfort of the driver in the tunnel or the like because of the needlessly changed operation mode toward a higher speed (e.g., to the HI mode). That is, a change of the operation mode, i.e., an increase of the wiping speed, is felt unnecessary to the driver in the tunnel when disturbance of visibility by the condensation on the windshield is not severe.